Photolithographic optics-based processes are used in the manufacture of integrated circuits, and since these processes require accurate focusing to produce a precise image, surface planarity becomes an important issue. This is becoming increasingly critical as line widths are being reduced in size in order to make semiconductor devices even more compact, and to provide higher speeds. More accurate optical focusing for finer line widths results in a loss of "depth of field"(i.e., the focusing is very accurate only in a plane of very limited depth). Accordingly, a planar surface is essential to ensure good focusing to enable the photolithographic process to produce fine line width, compact high speed semiconductor devices.
There are several techniques for planarizing the surface of a semiconductor wafer. One of these is chemical mechanical polishing (CMP). As indicated in an article entitled "Chemical Mechanical Polishing: The Future of Sub Half Micron Devices," Dr. Linton Salmon, Brigham Young University (Nov. 15, 1996), CMP is now considered the most effective method yet for planarizing wafers with sub micron lines. In this process, a wafer is mounted on a rotary carrier or chuck with the integrated circuit side facing outward. A polishing pad is then brought into contact with the integrated circuit side. Pressure may be applied by the carrier and/or the platen to effectuate polishing. According to Salmon, in some CMP machines the wafer rotates while the polishing pad is stationary, in others the pad rotates while the wafer carrier is stationary, and in yet another type both the wafer carrier and the pad rotate simultaneously. The polishing pad may be pre-soaked and continually re-wet with a slurry that has a variety of abrasive particles suspended in a solution. Typically, the particles range in size from 30 to 1,100 nanometers. After planarization through polishing, the wafers go through a post-CMP clean up to remove residual slurry, metal particles, and other potential contaminants from its surface.
An important variable in planarization through CMP is "removal rate" which is the rate of removal of material from the surface of the semiconductor wafer being polished. Preferably, the rate of removal should be such that any surface peaks are preferentially flattened and the resultant surface should be as near perfectly planar as possible. There are several factors that may affect the rate of removal. For example, the nature of the slurry can have a dramatic effect. The slurry includes abrasive particles suspended in a solvent which selectively may soften certain features of the pattern on the semiconductor wafer surface, thereby affecting the relative rate of removal of those features vis-a-vis others. As indicated in the above article, "The purpose of the slurry is simple, yet understanding and modeling all the mechanical and chemical reactions involved is nearly impossible." Accordingly, development of the CMP process has proceeded on a "trial and error basis."
Among the more advanced CMP machines presently available are the AvantGaard Model 776 of IPEC of Phoenix, Ariz. In this CMP apparatus, the lower head (containing the polishing pad) orbits, while the carrier holding the wafer rotates about a central axis. Polishing fluids (slurry) are introduced to the wafer directly through the polish pads with point-of-use mix, which results in better wafer uniformity and reduced slurry consumption.
There continues to be multiple challenges in CMP, making the polishing and planarization faster, more uniform across a wafer, and improving the variation seen in wafer to wafer results. The polishing motion of the pad and carrier play a crucial role in the CMP process along with the quality of the polishing pad over its life.
The polishing pad should be "conditioned" after a period of use to provide for a more uniform polishing rate, from wafer to wafer, and to provide for better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner arm with an abrasive lower surface is forced to come in contact with the pad upper surface while the pad oscillates and the conditioner arm moves back and forth in an arc about a pivot axis outside of the circumference of the polish pad. The combined pad oscillation and the conditioning arm arc motion during conditioning results in non-uniform pad surface removal and roughing. Areas closer to the arm arc pivot are conditioned at a higher rate than the areas more distant from the arc pivot. Over time, this non-uniform pad conditioning results in poorer polishing uniformity on the semiconductor wafers.
Semiconductor manufacturers consistently require CMP processes to improve over time. As semiconductor devices become ever more complex and device geometry becomes ever so much smaller, there exists a need to make the CMP removal rate more consistent from wafer to wafer and wafer lot to wafer lot, while also making the polishing results more uniform across the entire surface of a wafer. Furthermore, there is also a need for a method to provide better and more uniform conditioning of CMP pads during their lifetime.